Positive electrode active substance for lithium ion batteries, positive electrode for lithium ion batteries, and lithium ion battery

ABSTRACT

The present invention provides a positive electrode active material for a lithium ion battery which has high capacity and good rate characteristics. The positive electrode active material has a layer structure represented by the compositional formula: Li α (Ni β Me 1-β )O γ , wherein Me represents at least one type selected from the group consisting of Mn, Co, Al, Mg, Cr, Ti, Fe, Nb, Cu and Zr, x denotes a number from 0.9 to 1.2, y denotes a number from 0.5 to 0.65, and z denotes a number of 1.9 or more. The positive electrode active material is selected by measuring the coordinates of the lattice constant a and compositional ratio (Li/M) and selecting materials within the region enclosed by three lines given by the equations: y=−20.186x+59.079, y=35x−99.393, and y=−32.946x+95.78, wherein the x-axis represents a lattice constant a and the y-axis represents a compositional ratio (Li/M) of Li to M.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a positive electrode active materialfor lithium ion battery, a positive electrode for lithium ion battery,and a lithium ion battery.

2. Description of Related Art

A lithium-containing transition metal oxide is generally used as thepositive electrode active material of a lithium ion battery. Specificexamples of the lithium-containing transition metal oxide includelithium cobaltate (LiCoO₂), lithium nickelate (LiNiO₂), and lithiummanganate (LiMn₂O₄). The complexation of these metal oxides is undergoneto improve the characteristics (high capacity, cycle characteristics,preserving characteristics, reduction in internal resistance, and ratecharacteristics) and safety. The characteristics different from thoserequired for lithium ion batteries in mobile telephones and personalcomputers are required for lithium ion batteries used in large-sizedbattery applications such as car applications and road levelingapplications. Particularly, high capacity and rate characteristics areregarded as important.

Various methods have been used to attain high capacity and to improvethe rate characteristics. For example, Patent document 1 discloses alithium battery positive electrode made of a complex oxide representedby the general formula Li_(w)Ni_(x)Co_(y)Al_(z)O₂ (wherein w=0.90 to1.10, x=0.80 to 0.95, y=0.04 to 0.19, z=0.01 to 0.16, and x+y+z=1.0) andalso describes that a lithium battery positive electrode material, whichhas a large discharged capacity, is reduced in the deteriorations ofbattery characteristics caused by repetitive charge/discharge, issuperior in cycle characteristics, is limited in the generation of gascaused by the decomposition of a positive electrode material aftercharged, and improved in preservability/safety, can be provided.

Also, Patent document 2 discloses a complex oxide represented by thegeneral formula A_(w)D_(v)Ni_(x)Al_(y)N_(z)O₂ (wherein A represents atleast one type selected from alkali metals, D represents at least onetype selected from Mg and B, N represents at least one type selectedfrom Si, Ca, Cu, P, In, Sn, Mo, Nb, Y, Bi, and Ga, w, v, x, y, and zrespectively denote a number given by the following formulae:0.05≦w≦1.2, 0.001≦v≦0.2, 0.5≦x≦0.9, 0.1<y≦0.5, and 0.001≦z≦0.2) as apositive electrode active material in a battery which comprises anegative electrode, a positive electrode, and a nonaqueous electrolyteincluding a lithium salt and can be plurally charged/dischargedreversively. Patent document 2 also describes that this oxide enables asecondary battery positive electrode material to excel in all batterycharacteristics such as high capacitization, long life, ratecharacteristics, high-temperature characteristics, and safety.

-   (Patent document 1) Japanese Patent Application Publication No.    10-321224-   (Patent document 2) Japanese Patent Application Publication No.    10-208744

SUMMARY OF INVENTION

However, high capacitization and rate characteristics are importantcharacteristics required for a battery and there is a room forimprovement of a high-quality positive electrode active material forlithium ion battery.

In view of this situation, an object of the present invention is toprovide a positive electrode material for lithium ion battery having ahigh capacity and good rate characteristics.

The inventors have made earnest studies, and as a result, attractedtheir attentions to the relation between the lattice constant a of thepositive electrode active material, compositional ratio of Li to metals(M) other than Li, and battery characteristics, to find that a batteryproduced using the positive electrode active material has goodcharacteristics if the coordinates of a lattice constant a andcompositional ratio (Li/M) are within a predetermined region on a graphin which the x-axis represents the lattice constant a and the y-axisrepresents the compositional ratio (Li/M) of Li to M.

According to a first aspect of the present invention completed based onthe above teachings, there is provided a positive electrode activematerial for lithium ion battery which has a layer structure representedby the compositional formula: Li_(x)(Ni_(y)Me_(1-y))O_(z) (wherein Merepresents at least one type selected from the group consisting of Mn,Co, Al, Mg, Cr, Ti, Fe, Nb, Cu and Zr, x denotes a number from 0.9 to1.2, y denotes a number from 0.5 to 0.6 and z denotes a number of 1.9 ormore), wherein the coordinates of the lattice constant a andcompositional ratio (Li/M) are within the region enclosed by three linesgiven by the equations: y=−20.186x+59.079, y=35x−99.393, andy=−32.946x+95.78 on a graph in which the x-axis represents a latticeconstant a and the y-axis represents a compositional ratio (Li/M) of Lito M.

In an embodiment of the positive electrode active material for lithiumion battery according to the present invention, the coordinates of thelattice constant a and compositional ratio (Li/M) are within the regionenclosed by three lines given by the equations: y=−15x+44.18,y=33.33x−94.665, and y=−200x+575.92, and the lattice constant c is 14.2to 14.25.

In another embodiment of the positive electrode active material forlithium ion battery according to the present invention, M is one metalselected from the group consisting of Ni, Mn, and Co.

According to another aspect of the present invention, there is provideda positive electrode for lithium ion battery comprising the positiveelectrode active material according to the present invention.

According to a further aspect of the present invention, there isprovided a lithium ion battery comprising the positive electrode forlithium ion battery according to the present invention.

ADVANTAGEOUS EFFECT OF THE INVENTION

According to the present invention, a positive electrode active materialfor lithium ion battery which has a high capacity and good ratecharacteristics can be provided.

BRIEF DESCRIPTION OF DRAWING

FIG. 1 is a graph of “lattice constant a”-“compositional ratio of Li/M”according to an example.

DETAILED DESCRIPTION OF EMBODIMENTS

(Structure of Positive Electrode Active Material for Lithium IonBattery)

As the material of the positive electrode active material for lithiumion battery according to the present invention, compounds useful as thepositive electrode active material for the positive electrode of usuallithium ion batteries may be widely used. It is particularly preferableto use a lithium-containing transition metal oxide such as lithiumcobaltate (LiCoO₂), lithium nickelate (LiNiO₂), and lithium manganate(LiMn₂O₄). The positive electrode active material for lithium ionbattery which is produced using materials like the above has a layerstructure represented by the compositional formula:Li_(x)(Ni_(y)Me_(1-y))O_(z) (wherein Me represents at least one typeselected from Mn, Co, Al, Mg, Cr, Ti, Fe, Nb, Cu and Zr, x denotes anumber from 0.9 to 1.2, y denotes a number from 0.5 to 0.65, and zdenotes a number of 1.9 or more).

The ratio of lithium to the total metals in the positive electrodeactive material for lithium ion battery is 0.9 to 1.2. It is because astable crystal structure is scarcely kept when the ratio is less than0.9 whereas high capacity of the battery cannot be secured when theratio exceeds 1.2.

The positive electrode active material for lithium ion battery accordingto the present invention has the characteristics that the coordinates ofthe lattice constant a and compositional ratio (Li/M) are within theregion enclosed by three lines given by the equations:y=−20.186x+59.079, y=35x−99.393, and y=−32.946x+95.78 on a graph inwhich the x-axis represents the lattice constant a and the y-axisrepresents the compositional ratio (Li/M) of Li to M, and the latticeconstant c is 14.2 to 14.25. When the lattice constant c is 14.2 to14.25 and the coordinates of the lattice constant a and compositionalratio (Li/M) are within the above described region, the battery capacityusing the positive electrode active material can be increased and therate characteristics can be excellent.

Also, the coordinates of the lattice constant a and compositional ratio(Li/M) are preferably within a narrower region enclosed by three linesgiven by the equations: y=−15x+44.18, y=33.33x−94.665, andy=−200x+575.92, and the lattice constant c is preferably 14.22 to 14.25.

The positive electrode active material for lithium ion battery isconstituted of primary particles, secondary particles formed fromaggregated primary particles, or a mixture of primary particles andsecondary particles. The average particle diameter of these primary andsecondary particles of the positive electrode active material forlithium ion battery is preferably 2 to 8 μm.

When the average particle diameter is less than 2 μm, this makes itdifficult to apply the positive electrode active material to the currentcollector. When the average particle diameter exceeds 8 μm, voids areeasily produced when the active material particles are filled, leadingto less fillability. The average particle diameter is more preferably 3to 6 μm.

(Structure of Positive Electrode for Lithium Ion Battery and Lithium IonBattery Using Positive Electrode)

The positive electrode for lithium ion battery according to anembodiment of the present invention has a structure in which a positiveelectrode mix prepared by blending, for example, a positive electrodeactive material for lithium ion battery which has the aforementionedstructure, a conductive adjuvant, and a binder is applied to one or bothsurfaces of a current collector made of an aluminum foil or the like.Also, a lithium ion battery according to the embodiment of the presentinvention is provided with the positive electrode for lithium ionbattery having such a structure.

(Method for Producing Positive Electrode Active Material for Lithium IonBattery)

Next, a method for producing a positive electrode active material forlithium ion battery according to the embodiment of the present inventionwill be explained in detail.

First, a metal salt solution containing an oxidant is prepared. Themetal salt is a sulfate, chloride, nitrate, acetate, or the like and,particularly, a nitrate is preferable. This is because the nitrate canbe calcined as it is, so that a cleaning process can be omitted, even ifthe nitrate is mixed as impurities in the calcination raw material, andthe nitrate functions as an oxidant to promote oxidation of metals inthe calcination raw material. The metal contained in the metal salt isNi and at least one or more types selected from Mn, Co, Al, Mg, Cr, Ti,Fe, Nb, Cu, and Zr. As the nitrate of a metal, for example, nickelnitrate, cobalt nitrate, or manganese nitrate may be used. At this time,the metal salt is prepared such that each metal is contained in adesired molar ratio. The molar ratio of each metal in the positiveelectrode active material is thereby determined.

Next, lithium carbonate is suspended in pure water, and then, a metalsalt solution of the above metal is poured into the mixture to produce alithium salt solution slurry. At this time, lithium-containing carbonatemicroparticles precipitate in the slurry. In this case, a sulfate orchloride is washed with a saturated lithium carbonate solution and thenseparated by filtration when the lithium compound does not react withthe metal salt in the heat-treatment. When, like the case of using anitrate or acetate, the lithium compound reacts as the lithium rawmaterial during heat treatment, these metal salts are not washed andseparated as it is by filtration, followed by drying, thereby enablingthe salt to be used as a calcination precursor.

Next, the separated lithium-containing carbonate is dried to obtain alithium salt composite (precursor of a positive electrode activematerial for lithium ion battery) powder.

Next, a sagger having a predetermined capacity is prepared and thepowder of the precursor of a positive electrode active material forlithium ion battery is filled in the sagger. Next, the sagger filledwith the powder of the precursor of the positive electrode activematerial for lithium ion battery is transferred to a kiln to calcine.The calcination is performed by keeping the sagger with heating for apredetermined time in an oxygen atmosphere. Also, it is desirable thatthe calcination is performed under a pressure of 101 to 202 KPa becausethe quantity of oxygen in the composition is increased. The calcinationtemperature is 700 to 1100° C., and the calcination is carried outpreferably at 700 to 950° C. when y in the above formula satisfies theequation: 0<y≦0.5 and at 850 to 1100° C. when y in the above formulasatisfies the equation: 0.5<y≦0.9. The crystallinity of the positiveelectrode active material is largely caused by the relation between thecomposition and calcination temperature. At this time, there is the casewhere even a small difference in composition affects the crystallinityof the positive electrode active material though depending on the rangeof calcination temperature. When the positive electrode active materialprecursor is made to have a proper compositional ratio and calcined at aproper calcination temperature corresponding to the compositional ratio,the crystallinity of the positive electrode active material is improvedto make a high-performance positive electrode active material. Also, thecrystallinity of the positive electrode active material is affected notonly by the above factor but also by the grain size of the precursor andthe amount of lithium carbonate used as the raw material. When theamount of lithium carbonate is large and a lot of lithium is containedin the positive electrode material precursor, the calcination proceedssmoothly. In this case, the lattice constant c is decreased withincrease in calcination temperature whereas the lattice constant c isincreased with decrease in calcination temperature because thecalcination is insufficient.

After that, the powder is taken out of the sagger and ground to obtain apositive electrode active material powder.

In this case, when a nitrate is used as the metal salt to be poured inthe production of the lithium salt solution slurry, a positive electrodeactive material containing oxygen exceeding that in the compositionalformula is finally produced. Also, when the calcination of the positiveelectrode precursor is performed not under atmospheric pressure butunder a predetermined pressure, a positive electrode active materialcontaining oxygen exceeding that in the compositional formula is finallyproduced. When the positive electrode active material contains oxygenexceeding that in the compositional formula as mentioned above, abattery using the positive electrode active material is improved invarious characteristics.

EXAMPLES

Although examples are provided for facilitating understanding of thepresent invention and its advantage, the present invention is notlimited to the following examples.

Examples 1 to 29

First, lithium carbonate to be charged in an amount as described inTable 1 was suspended in 3.2 liter of pure water, and then, 4.8 liter ofa metal salt solution was added to the mixture. Here, the metal saltsolution was prepared in such a manner that the compositional ratio of ahydrate of a nitrate of each metal was that described in Table 1 and thenumber of moles of all metals was 14.

In this case, the amount of lithium carbonate to be suspended is a valueat which x in the formula Li_(x)(Ni_(y)Me_(1-y))O_(z) of a product(positive electrode for lithium ion secondary battery, that is, positiveelectrode active material) accords to that described in Table 1 and iscalculated according to the following equation.W(g)=73.9×14×(1+0.5X)×A

In the above formula, “A” is a value multiplied in order to subtract, inadvance, the amount of lithium originated from a lithium compound otherthan lithium carbonate left in the raw material after filtration besidesthe amount required for the precipitation reaction. “A” is 0.9 when,like the case of using a nitrate or acetate, the lithium salt reacts asthe calcination raw material, and 1.0 when, like the case of using asulfate or chloride, the lithium salt does not react as the calcinationraw material.

Though lithium-containing carbonate microparticles were precipitated inthe solution by this treatment, this precipitate was separated byfiltration using a filter press.

Subsequently, the precipitate was dried to obtain a lithium-containingcarbonate (precursor of positive electrode active material for lithiumion battery).

Next, a sagger was prepared to fill the lithium-containing carbonatetherein. Next, the sagger was placed in an oxygen ambient furnace underatmospheric pressure and heated to 800 to 940° C. for 4 hr. Then, thesagger was kept at this temperature under heating for 12 to 30 hr andthen, allowed to cool for 3 hr to obtain an oxide. Then, the obtainedoxide was pulverized to obtain a positive electrode active materialpowder for lithium ion battery.

Example 30

In Example 30, the same procedures as in Examples 1 to 29 were carriedout except that each metal of the raw material was altered to thecomposition shown in Table 1, and a chloride was used as the metal saltto precipitate a lithium-containing carbonate, which was then washedwith a saturated lithium carbonate solution, followed by filtration.

Example 31

In Example 31, the same procedures as in Examples 1 to 29 were carriedout except that each metal of the raw material was altered to thecomposition shown in Table 1 and a sulfate was used as the metal salt toprecipitate a lithium-containing carbonate, which was then washed with asaturated lithium carbonate solution, followed by filtration.

Example 32

In Example 32, the same procedures as in Examples 1 to 29 were carriedout except that each metal of the raw material was altered to thecomposition shown in Table 1 and the calcination was performed not underatmospheric pressure but under a pressure of 120 KPa.

Comparative Examples 1 to 22

In Comparative Examples 1 to 22, the same procedures as in Examples 1 to29 were carried out except that each metal in the raw material wasaltered to the composition shown in Table 1.

TABLE 1 Li₂CO₃ calcination suspension compositional ratio (%) of eachmetal in all metals except Li temperature solution(g) Ni Co Mn Ti Cr FeCu Al Sn Mg (° C.) Example 1 1397 55 25 20 900 2 1406 55 25 20 850 31397 60 15 25 800 4 1397 60 15 20 2.5 2.5 850 5 1397 60 15 20 2.5 2.5850 6 1397 60 15 20 5 800 7 1397 60 15 20 5 800 8 1397 60 15 20 5 800 91397 60 15 20 5 800 10 1397 60 15 20 5 800 11 1397 60 15 20 5 820 121406 60 15 20 5 820 13 1415 60 15 20 5 820 14 1406 50 30 20 880 15 139750 30 20 860 16 1397 65 20 15 820 17 1397 65 25 5 2.5 2.5 850 18 1397 6525 5 5 850 19 1397 65 25 5 5 850 20 1397 65 25 5 5 850 21 1397 65 25 5 5850 22 1397 65 25 5 5 850 23 1397 65 25 5 5 850 24 1397 60 30 5 2.5 2.5850 25 1397 60 30 5 5 850 26 1397 60 30 10 860 27 1406 60 30 10 860 281397 60 20 20 860 29 1397 60 20 15 5 880 30 1500 60 15 25 820 31 1500 6015 25 800 32 1397 60 15 25 820 Compar- 1 1397 55 25 20 940 ative 2 140655 25 20 880 Example 3 1397 60 15 25 830 4 1406 60 15 25 860 5 1415 6015 25 860 6 1397 60 15 20 2.5 2.5 820 7 1397 60 15 20 5 820 8 1397 60 1520 5 820 9 1397 60 15 20 5 820 10 1397 60 15 20 5 820 11 1397 60 15 20 5820 12 1397 60 15 20 5 840 13 1397 60 15 20 5 840 14 1397 50 30 20 90015 1406 60 30 10 870 16 1397 65 25 5 2.5 2.5 870 17 1397 65 25 5 5 87018 1397 65 25 5 5 870 19 1393 65 25 5 5 870 20 1393 65 25 5 5 870 211397 50 30 20 890 22 1397 60 15 25 820(Evaluation)

The contents of Li, Ni, Mn, and Co in each positive electrode activematerial were measured by induction coupling plasma atomic emissionspectrometry (ICP-AES) to calculate the compositional ratio (molarratio) of each metal. Also, the crystal structure was confirmed to be alayer structure by X-ray diffraction.

Moreover, each positive electrode material was measured by powder XRDdiffraction to find the lattice constant from the diffraction pattern.Also, among the measured factors, the lattice constant a was made to lieon the x-axis and the compositional ratio (Li/M) of Li to M (all metalsexcluding Li) found from MS analysis was made to lie on the y-axis todraw a graph as shown in FIG. 1.

The positive electrode material, a conductive material, and a binderwere weighed in a ratio of 85:8:7. The positive electrode activematerial and the conductive material were mixed in a solution preparedby dissolving the binder in an organic solvent (N-methylpyrrolidone)into a slurry, which was then applied to the surface of an Al foil andpressed after dried to produce a positive electrode. In succession, a2032-type coin cell for evaluation in which Li was used as the counterelectrode was manufactured and an electrolytic solution prepared bydissolving 1M-LiPF₆ in EC-DMC (1:1) was used. Then, the coin cellimpregnated with the electrolytic solution was used to calculate thebattery capacity ratio of a battery capacity at 1 C to a batterycapacity at 0.2 C to obtain the rate characteristics of the battery.These results are shown in Table 2.

TABLE 2 discharged rate lattice lattice lattice capacity(0.2 C)characteristics constant a constant c volume v Li/M (A · h/g) (%)Example 1 2.8721 14.2318 101.666 1.131 171 88 2 2.8725 14.2331 101.7101.145 171 88 3 2.8735 14.2374 101.808 1.142 170 88 4 2.8736 14.2382101.821 1.116 171 88 5 2.8743 14.2383 101.875 1.099 171 88 6 2.874114.2393 101.861 1.096 171 89 7 2.8733 14.2326 101.761 1.085 173 88 82.8739 14.2412 101.865 1.089 171 88 9 2.8733 14.2350 101.774 1.087 17288 10 2.8741 14.2390 101.862 1.095 172 89 11 2.8739 14.2383 101.8431.095 174 89 12 2.8733 14.2361 101.781 1.086 172 89 13 2.8742 14.2371101.854 1.076 172 89 14 2.8735 14.2352 101.793 1.078 173 88 15 2.873114.2335 101.751 1.105 176 89 16 2.8755 14.2431 101.991 1.059 173 88 172.8741 14.2408 101.875 1.088 171 88 18 2.8734 14.2277 101.734 1.115 18088 19 2.8730 14.2257 101.690 1.085 172 88 20 2.8727 14.2224 101.6441.087 171 88 21 2.8734 14.2313 101.761 1.079 179 88 22 2.8730 14.2293101.711 1.089 175 88 23 2.8742 14.2280 101.787 1.067 180 88 24 2.871714.2383 101.889 1.110 170 88 25 2.8761 14.2480 102.072 1.025 171 88 262.8755 14.2463 102.014 1.056 171 88 27 2.8758 14.2462 102.032 1.036 17088 28 2.8749 14.2460 101.970 1.045 171 88 29 2.8749 14.2436 101.950 1.08170 88 30 2.8728 14.2304 101.710 1.067 165 86 31 2.8755 14.2314 101.9091.081 162 85 32 2.8736 14.2354 101.782 1.09 178 90 Compar- 1 2.876214.2480 102.072 1.064 173 87 ative 2 2.8724 14.2318 101.690 1.156 166 89Example 3 2.8714 14.2205 101.540 1.079 175 87 4 2.8724 14.2318 101.6901.156 166 89 5 2.8735 14.2408 101.833 1.069 167 86 6 2.8741 14.2373101.852 1.127 167 85 7 2.8732 14.2386 101.792 1.163 163 82 8 2.876214.2526 102.096 1.000 160 83 9 2.8752 14.2562 102.131 1.022 163 85 102.8761 14.2441 102.039 1.016 170 87 11 2.8772 14.2500 102.161 1.008 16787 12 2.8721 14.2326 101.677 1.086 172 88 13 2.8711 14.2216 101.5271.108 171 86 14 2.8715 14.2259 101.617 1.122 169 87 15 2.8748 14.2402101.908 1.105 173 87 16 2.8756 14.2362 101.875 1.068 171 86 17 2.874514.2326 101.771 1.040 171 86 18 2.8772 14.2500 102.161 1.028 167 87 192.8755 14.2402 101.970 1.094 171 87 20 2.8728 14.2304 101.710 1.067 16583 21 2.8734 14.1915 101.598 1.056 161 83 22 2.8754 14.2521 101.9951.061 163 82

When the resultant data of Table 2 is plotted on a graph of FIG. 1 andlinear lines are drawn so as to enclose the plotted points at which goodcapacity and rate characteristics of the battery are exhibited, it isfound that these points are included within the region enclosed by threelines: (1) y=−20.186x+59.079, (2) y=35x−99.393, and (3) y=−32.946x+95.78in FIG. 1.

Generally, considerable time is required to evaluate batterycharacteristics when a positive electrode active material is used for abattery. However, according to the present invention, thecharacteristics of a battery provided with a positive electrode activematerial having a predetermined lattice constant c can be evaluated onlyby writing the above three lines on a graph in which the x-axisrepresents the lattice constant a and the y-axis represents thecompositional ratio (Li/M) of Li to M to determine whether or not thecoordinates of the lattice constant a and compositional ratio (Li/M) arewithin the region enclosed by the three lines. Therefore, the timerequired for the evaluation of a battery can be shortened, whichimproves the efficiency of battery production and reduces productioncost.

Moreover, it is found that if lines are drawn so as to enclose thosehaving more excellent battery capacity and rate characteristics, theyare included within the region enclosed by three lines: (1)y=−15x+44.18, (2) y=33.33x−94.665, and (3) y=−200x+575.92 in FIG. 1.

Also, Examples 1 to 29 and 32 each use a nitrate as the metal salt to bepoured and therefore, positive electrode active materials containingoxygen exceeding that in the compositional formula is finally produced.When comparing Examples 30 and 31 using a chloride and sulfate as themetal salt with those having the same condition other than the metalsalt, the battery characteristics are more improved (for example,comparison of Example 3 with Examples 30 and 31).

Moreover, in Example 32 in which the positive electrode materialprecursor was calcined not under atmospheric pressure but under apredetermined pressure, a positive electrode active material containingoxygen exceeding that in the compositional formula is finally produced.Therefore, when comparing Example 32 with those having the samecondition other than the pressure, Example 32 was more improved inbattery characteristics (for example, comparison of Example 3 withExample 32).

Comparative Examples 1 to 20 are excluded from the region enclosed bythree lines: (1) y=−20.186x+59.079, (2) y=35x−99.393, and (3)y=−32.946x+95.78 in FIG. 1 and had inferior battery characteristics.Also, Comparative Examples 21 and 22 had inferior batterycharacteristics because they had a lattice constant c out of the definedrange from 14.2 to 14.25, though they were included within the regionenclosed by three lines: (1) y=−20.186x+59.079, (2) y=35x−99.393, and(3) y=−32.946x+95.78 in FIG. 1.

What is claimed is:
 1. A positive electrode active material for alithium ion battery which has a layer structure represented by thecompositional formula:Li_(α)(Ni_(β)Me_(1-β))O_(γ), wherein Me represents at least one typeselected from the group consisting of Mn, Co, Al, Mg, Cr, Ti, Fe, Nb, Cuand Zr, α denotes a number from 0.9 to 1.2, β denotes a number from 0.5to 0.65, and γ denotes a number of 1.9 or more, wherein the measuredcoordinates of the lattice constant a and compositional ratio (Li/M),wherein M is the sum of all metals excluding Li in the abovecompositional formula, are within the region enclosed by three linesgiven by the equations: y=−20.186x +59.079, y=35x −99.393, andy=−32.946x +95.78 on a graph in which the x-axis represents a latticeconstant a and the y-axis represents a compositional ratio (Li/M) of Lito M, and the lattice constant c is 14.2 to 14.25.
 2. The positiveelectrode active material for a lithium ion battery according to claim1, wherein the coordinates of the lattice constant a and compositionalratio (Li/M) are within the region enclosed by three lines given by theequations: y=−15x+44.18, y=33.33x−94.665, and y=−200x+575.92.
 3. Thepositive electrode active material for a lithium ion battery accordingto claim 1, wherein Me represents one metal selected from the groupconsisting of Mn and Co.
 4. A positive electrode for lithium ion batterycomprising the positive electrode active material of claim
 1. 5. Alithium ion battery comprising the positive electrode for lithium ionbattery of claim
 4. 6. A positive electrode for lithium ion batterycomprising the positive electrode active material of claim
 2. 7. Apositive electrode for lithium ion battery comprising the positiveelectrode active material of claim
 3. 8. A lithium ion batterycomprising the positive electrode for lithium ion battery of claim
 6. 9.A lithium ion battery comprising the positive electrode for lithium ionbattery of claim 7.